1. Field of the Invention
This invention relates to offshore petroleum production. More particularly, it relates to the connection of subsea steel catenary risers to floating production, storage and offloading vessels (FPSO's) and the like.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98.
A riser is a pipe or assembly of pipes used to transfer produced fluids from the seabed to the surface facilities or to transfer injection fluids, control fluids or lift gas from the surface facilities and the seabed. An SCR (Steel Catenary Riser) is a deepwater steel riser suspended in a single catenary from a platform (typically a floater) and connected horizontally on the seabed.
In ultra deepwater, riser systems become a technical challenge and a major part of the field development costs. Large external pressures in these great depths cause flexible solutions to run into weight and cost problems. These same depths however enable steel pipe configurations to maintain curvatures that cause little bending and thus make them suitable for deepwater SCR use.
U.S. Pat. No. 7,472,755 to Riggs discloses a method for servicing a component of a riser system, such as a flexible joint. The riser system may be supported in a support apparatus such that the flexible joint and an adjoining section of riser are detached to allow for inspection, servicing, repair, and/or replacement of the flexible joint or various subcomponents thereof. An apparatus is also disclosed for supporting the flexible joint during servicing.
The steel catenary riser (SCR) concept has recently been used in almost every new deepwater field development around the world. Perhaps the first implementation of the SCR concept occurred in 1994 on the Shell Oil Company's “Auger” tension leg platform (TLP) in 872 m (2860 ft) water depth. Since then, SCR's have been vital to deepwater field developments. Their use has given a new dimension to oil exploration and transportation in water depths where other riser concepts could not tolerate the environmental loads or would have become very costly. SCR designs are very sensitive to floating support platform or vessel motion characteristics to which they are typically attached. In addition to pipe stresses, the main design issue for the SCR concept is fatigue related. There are two main sources for fatigue: random wave fatigue and vortex-induced vibration (VIV) fatigue. The former is due to wave action and the associated platform motion characteristics. The VIV fatigue is mainly due to current conditions.
In the past, an FPSO having a large displacement has typically been used to carry a large number of these deepwater SCR's. Concerns relating to SCR bending fatigue in this use have been addressed and shown not to be a problem in mild environments.
In less benign metocean conditions, a Buoyancy Supported Riser System (BSR) and Sub-Surface Buoy (SSB) may be used to locate the upper terminus of the SCR below the zone affected by wind and waves. In this way, overall motion of the SCR is reduced, leading to decreased wear and metal fatigue.